1. Field of the Invention
The invention relates in general to video moire reduction and in particular to a method and device for preventing the production of the second harmonic of the video signal Nyquist frequency.
2. Description of the Prior Art
Moire is a common word for patterns which look like waves on water. An example in the real world is the interference between two fences on a bridge, but it is also easily shown with simple patterns. Moire appears when two sampling processes don't match each other and there is no sufficient filtering (interpolation/decimation).
An analog low pass filter can be used to reduce the second harmonics, as shown in U.S Pat. No. 5,729,301 to Sluyterman, having a pass-band up to the Nyquist frequency and a stop-band before the second harmonic. The low pass filter in the '301 patent is placed after an inverse gamma function for linear (voltage proportional to luminance) signals, and after low pass filtering, the signal is converted back to the incoming nonlinear gamma characteristic. The second harmonic in this example is removed by the low pass filter. FIG. 2 shows an example of this placement of the analog low pass filter 2 in the prior art. If the low pass filter 2 is not present, then the nonlinear function 3a and the inverse nonlinear function 3b would merely operate to cancel each other's affects and the input video signal to the nonlinear function 3a would be equivalent to the displayed video signal and include all higher harmonics. This low pass filter 2 must be of a higher order especially when used for the high pixel frequencies of computer monitors. This low pass filter 2, however, is costly and reduces the sharpness of the picture.
Typically, alias frequencies appear when the repetition spectrum of the first carrier (the closest carrier to the baseband video signal) overlaps with the baseband. When the alias frequency occurs within the baseband bandwidth the moire patterns caused are not removable without loss of signal information.
Aliasing also occurs when the horizontal pixel sampling frequency is close to the sampling frequency of the shadow mask as explained below. In this case the second harmonic of the video signal Nyquist frequency (half of the pixel sampling frequency) produces the most aliasing.
The shadow mask has a first horizontal sampling frequency. This first sampling frequency should not come close to the input video signal pixel frequency or higher harmonics of the video signal. However, it is common to use a shadow mask where its sampling frequency almost matches the pixel frequency of the graphics format. If an alternating pixel on/off pattern of the video signal is in the range of the Nyquist frequency of the shadow mask the second harmonic of the video signal produces aliasing.
This second harmonic, and even higher harmonics, is created when a nonlinear function is applied to the video signal. A nonlinear function typically occurs in the CRT gun itself and is known as the gamma characteristic. The gamma characteristic is precorrected for by the camera which applies a nonlinear inverse gamma function to the signal before transmission. This precorrection assures that the resulting luminance of the display has a linear relation to the scene luminance of the video signal. In the case of computer graphics, most graphics are generated with respect to the CRT characteristics. The gamma characteristic of a CRT is a power function between the electrical amplitude U.sub.in and the luminance L: EQU L=k(U.sub.in).sup..gamma.
The .gamma. value for TVT's is usually defined to be 2.2 but .gamma. is tube dependent and ranges from 2.2 to 2.5. The effect of a nonlinear process, such as the gamma characteristic of a CRT, is shown in FIGS. 1e-g by signal s.sub.g (n). For the normalized input amplitudes 0 and 1, the output signal is identical; therefore, bursts with amplitudes close to 0 and 1 will not change. But all other amplitudes are changed due to the gamma characteristic. This causes higher harmonics (2f.sub.0, etc.), as shown in the spectrum .vertline.S.sub.g (f).vertline. of FIG. 1g. The repetition frequency f.sub.s -2f.sub.0 is a low alias frequency, created by the carrier f.sub.s. For illustration purposes, a low pass filter applied to the signal shows how it can remove the signal frequency f.sub.0 and the repetition frequency f.sub.s -f.sub.0, but not the alias frequency f.sub.s -2f.sub.0. The output signal contains the DC level and the alias frequency. FIGS. 1a-d show the signal without application of a nonlinear function.
An analog low pass filter can be used to reduce the second harmonics, as shown in Ser. No. 08/707,452, now U.S. Pat. No. 5,729,301, March 1998 having a pass-band up to the Nyquist frequency and a stop-band before the second harmonic. The low pass filter in Ser. No. 08/707,452 now U.S. Pat. No. 5,729,301 issued March 1998 is placed after an inverse gamma function for linear (voltage proportional to luminance) signals, and after low pass filtering, the signal is converted back to the incoming nonlinear gamma characteristic. The second harmonic in this example is removed by the low pass filter. FIG. 2 shows an example of this placement of the analog low pass filter 2 in the prior art. If the low pass filter 2 is not present, then the nonlinear function 3a and the inverse non-linear function 3b would merely operate to cancel each other's affects and the input video signal to the nonlinear function 3a would be equivalent to the displayed video signal and include all higher harmonics. This low pass filter 2 must be of a higher order especially when used for the high pixel frequencies of computer monitors. This low pass filter 2, however, is costly and reduces the sharpness of the picture.